Removeable embolus blood clot filter and filter delivery unit

ABSTRACT

A blood clot filter which is collapsible toward a central longitudinal axis into a collapsed configuration for insertion into a blood vessel and which is radially expandable outwardly from the longitudinal axis to an expanded configuration for contact with the inner wall of the blood vessel at two longitudinal spaced locations. A first plurality of spaced, elongate arms, in the expanded configuration of the filter, curve outwardly away from the longitudinal axis toward the leading end of the filter to form a first filter basket and to center a hub at the trailing end of the filter within the vessel. A second plurality of spaced elongate legs angle outwardly away from the longitudinal axis toward the leading edge of the filter in the expanded configuration thereof to form a second filter basket opening toward the leading end. The ends of these legs include hooks to bend and straighten in response to withdrawal force.

PRIORITY

This application is a continuation of U.S. patent application Ser. No.09/640,865, filed on Aug. 18, 2000, now U.S. Pat. No. 7,314,477, whichis a division of U.S. patent application Ser. No. 09/360,654, filed onJul. 26, 1999, now U.S. Pat. No. 6,258,026, which is acontinuation-in-part of U.S. patent application Ser. No. 09/160,384,filed on Sep. 25, 1998, now U.S. Pat. No. 6,007,558. Each of thepreviously mentioned applications and patents is incorporated byreference in its entirety into this application.

BACKGROUND OF THE INVENTION

In recent years, a number of medical devices have been designed whichare adapted for compression into a small size to facilitate introductioninto a vascular passageway and which are subsequently expandable intocontact with the walls of the passageway. These devices, among others,include blood clot filters which expand and are held in position byengagement with the inner wall of a vein. It has been found to beadvantageous to form such devices of a shape memory material having afirst, relatively pliable low temperature condition and a second,relatively rigid

high-temperature condition. By forming such devices of temperatureresponsive material, the device in a flexible and reduced stress statemay be compressed and fit within the bore of a delivery catheter whenexposed to a temperature below a predetermined transition temperature,but at temperatures at or above the transition temperature, the deviceexpands and becomes relatively rigid.

Known self expanding medical devices have been formed of Nitinol, analloy of titanium and nickel which provides the device with a thermalmemory. The unique characteristic of this alloy is its thermallytriggered shape memory, which allows a device constructed of the alloyto be cooled below a temperature transformation level to a martensiticstate and thereby softened for loading into a catheter in a relativelycompressed and elongated state, and to regain the memorized shape in anaustenitic state when warmed to a selected temperature above thetemperature transformation level, such as human body temperature. Thetwo interchangeable shapes are possible because of the two distinctmicrocrystalline structures that are interchangeable with a smallvariation in temperature. The temperature at which the device assumesits first configuration may be varied within wide limits by changing thecomposition of the alloy. Thus, while for human use the alloy may befocused on a transition temperature range close to 98.6.degree. F., thealloy readily may be modified for use in animals with different bodytemperatures.

U.S. Pat. No. 4,425,908 to Simon discloses a very effective blood clotfilter formed of thermal shape memory material. This filter, like mostpreviously developed vena cava filters such as those also shown by U.S.Pat. Nos. 5,108,418 to Lefebvre, 5,133,733 to Rasmussen et al.,5,242,462 to El-Nounou et al., 5,800,457 to Gelbfish and 5,853,420 toChevillon et al. is a permanent filter which, when once implanted, isdesigned to remain in place. Such filters include structure to anchorthe filter in place within the vena cava, such as elongate diverginglegs with hooked ends that penetrate the vessel wall and positivelyprevent migration in either direction longitudinally of the vessel. Thehooks on filters of this type are rigid and will not bend, and withintwo to six weeks after a filter of this type has been implanted, theendothelium layer grows over the diverging legs and positively locks thehooks in place. Now any attempt to remove the filter results in a riskof injury to or rupture of the vena cava.

A number of medical procedures subject the patient to a short term riskof pulmonary embolism which can be alleviated by a filter implant. Insuch cases, patients are often adverse to receiving a permanent implant,for the risk of pulmonary embolism may disappear after a period ofseveral weeks or months. However, most existing filters are not easilyor safely removable after they have remained in place for more than twoweeks, and consequently longer term temporary filters which do notresult in the likelihood of injury to the vessel wall upon removal arenot available.

In an attempt to provide a removable filter, two filter baskets havebeen formed along a central shaft which are conical in configuration,with each basket being formed by spaced struts radiating outwardly froma central hub for the basket. The central hubs are held apart by acompression unit, and the arms of the two baskets overlap so that thebaskets face one another. Devices of this type require the use of tworemoval devices inserted at each end of the filter to draw the basketsapart and fracture the compression unit. The end sections of the armsare formed to lie in substantially parallel relationship to the vesselwall and the tips are inclined inwardly to preclude vessel wallpenetration. If a device of this type is withdrawn before theendothelium layer grows over the arms, vessel wall damage is minimized.However, after growth of the endothelium layer the combined inward andlongitudinal movement of the filter sections as they are drawn apart cantear this layer. U.S. Pat. No. 5,370,657 to Irie is illustrative of aprior art removable filter of this type which requires two removaldevices.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide a vesselimplantable filter of shape memory material having temperature inducedaustenitic and martensite states which maybe easily removed by a singleremoval device after an extended period of time without significantlyinjuring the vessel wall.

Another object of the present invention is to provide a blood clotfilter of shape memory material which operates in a temperature inducedaustenitic state to exert a force on the wall of a vessel by means ofoppositely disposed legs to maintain the filter in place, but which mayeasily be removed after the endothelium layer has covered the ends ofthe filter legs without significant damage to the vessel wall.

A further object of the present invention is to provide a novel andimproved vessel implantable filter having a group of arms and a group oflegs which incline from a central axis. The ends of the arms in thegroup of arms are oriented to engage a vessel wall to orient and centerthe filter in the vessel, and the ends of the legs of the group of legsare oriented to engage the vessel wall to prevent longitudinal movementof the filter along the vessel. The ends of at least some of the legsare provided with hooks configured to be more elastic than the legs topermit the hooks to straighten in response to a withdrawal force tofacilitate withdrawal from the endothelium layer without risk ofsignificant injury to the vessel wall. In some cases, similar hooks canbe formed on the ends of at least some of the arms.

Yet another object of the present invention is to provide a novel andimproved vessel implantable filter having one or more expandableappendages which engage the wall of the vessel. An elastic hook isformed on the free end of an appendage to pierce the vessel wall andinsure that the filter does not migrate in response to normalrespiratory functions or in the event of a massive pulmonary embolism.The hook is formed to have a maximum migration force, and when subjectedto forces below the maximum migration force, the hook retains its shape.When subjected to forces above the maximum migration force, the hookstraightens and can be withdrawn without significant damage to thevessel wall.

A further object of the present invention is to provide a novel andimproved vessel implantable filter having a plurality of expandableappendages which engage the wall of a vessel. Three to twelve of suchappendages are provided which have an elastic hook formed on the freeend of the appendage to pierce the vessel wall and insure that thefilter does not migrate when subjected to a pressure gradient fallingwithin a range of from 10 mmHg to 120 mmHg in a 28 mm vessel (filtermigration resistance). Each hook is formed to have a maximum migrationforce, and when subjected to forces below the maximum migration force,the hook retains its shape. When subjected to forces above the maximummigration force, the hook straightens and can be withdrawn withoutsignificant damage to the vessel wall. The maximum migration force foreach hook is dependent upon the desired total filter migrationresistance and the number of hooks formed on the filter.

A still further object of the present invention is to provide a noveland improved removable embolus blood clot filter and filter deliveryunit designed to insure delivery of the filter in a centered orientationto a precise location within a vessel. The filter delivery unit includesan elongate pusher wire of shape memory material having temperatureinduced austenitic and martensite states, with a handle at one end and afilter engaging spline at the opposite end. Spaced inwardly from thespline is a pusher pad which is longitudinally slotted to receive theelongate appendages of the filter. The pusher wire is reduced indiameter between the spline and pusher pad at a point adjacent to thepusher pad to impart a directional hinge to the pusher wire at thereduced portion.

According to the invention, a resilient blood clot filter is inwardlyradially collapsible toward its longitudinal axis into a collapsedconfiguration for insertion into a body vessel, but is adapted forautomatic radial expansion into contact with the inner wall of thevessel at two longitudinally spaced peripheral locations therein. Thefilter has leading and trailing ends and comprises a plurality of wires.The wires, in the normal expanded configuration of the filter, are inthe form of a plurality of elongated arms and legs with openings betweenthe wires to provide filter baskets opening at the leading end of thefilter. The wires have peripheral portions for contact with the innerwall of the vein at two longitudinally spaced peripheral locations. Thearms operate to center the filter while the legs terminate in hookswhich anchor the filter but which straighten in response to forceapplied at the trailing end of the filter to facilitate removal of thefilter.

To provide a filter that is inwardly radially collapsible from itsnormally expanded configuration toward its longitudinal axis into acollapsed configuration for insertion into a body vessel, the blood clotfilter is preferably formed from a plurality of wire portions composedof a thermal shape memory material having a first, low-temperaturecondition and a second, high-temperature condition. The material in itslow-temperature condition is relatively pliable (so that the wireportions may be straightened) and in its high-temperature condition isresiliently deformable and relatively rigid, and takes a predeterminedfunctional form.

In the high-temperature condition of the material, the filter comprisescoaxial first and second filter baskets, each filter basket beinggenerally symmetrical about the longitudinal axis of the filter withboth filter baskets being concave relative to the filter leading end.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a view in side elevation of an expanded blood clot filter ofthe present invention;

FIG. 2 is a view in side elevation of a hook for a leg of the filter ofFIG. 1;

FIG. 3 is a view in side elevation of a second embodiment of a hook fora leg of the filter of FIG. 1;

FIG. 4 is a cross sectional view of the blood clot filter of the presentinvention in place in a blood vessel;

FIG. 5 is a diagrammatic view of a second embodiment of the legstructure for the blood clot filter of the present invention;

FIG. 6 is a plan view of the filter delivery unit of the presentinvention;

FIG. 7 is an enlarged view in end elevation of the pusher pad for thefilter delivery unit of FIG. 6; and

FIG. 8 is an enlarged view of the end section of the filter deliveryunit of FIG. 6 in engagement with a filter.

DETAILED DESCRIPTION

By forming the body of a blood clot filter of a Nitinol alloy material,such as Nitinol wire, transition between the martensitic and austeniticstates of the material can be achieved by temperature transitions aboveand below a transition temperature or transition temperature range whichis at or below body temperature. Such controlled temperature transitionshave conventionally been employed to soften and contract the Nitinolfilter body to facilitate insertion into a catheter and to subsequentlyexpand and rigidify the body within a vascular or other passageway.Although the filters of the present invention are preferably formed froma temperature responsive shape memory material, such as Nitinol, theycan also be formed of a compressible spring metal such as stainlesssteel or a suitable plastic.

Referring now to FIG. 1, an expanded blood clot filter 10 is illustratedwhich is made from sets of elongate metal wires. The wires are heldtogether at the filter trailing end by a hub 12 where they are plasmawelded together and to the hub or otherwise joined. In the lowtemperature martensite phase of wires made of thermal shape memorymaterial, the sets of wires can be straightened and held in a straightform that can pass through a length of fine plastic tubing with aninternal diameter of approximately 2 mm (#8 French catheter). In itshigh temperature austenitic form, the filter 10 recovers a preformedfiltering shape as illustrated by FIG. 1. Similarly, wires of springmetal can be straightened and compressed within a catheter or tube andwill diverge into the filter shape of FIG. 1 when the tube is removed.

In its normal expanded configuration or preformed filtering shape,filter 10 is a double filter, having a first forwardly disposed filterbasket section 14 at the forward or leading end of the filter and asecond forwardly disposed filter basket section 16. The two filterbasket sections provide peripheral portions which can both engage theinner wall of a body vessel 17 at two longitudinally spaced locations,and the two filter basket sections are generally symmetrical about alongitudinal axis passing through the hub 12. On the other hand, thesecond forwardly disposed filter basket section 16, which is primarily acentering unit, may not always touch the vessel wall on all sides.

The second filter basket section 16 is formed from short lengths of wirewhich form arms 18 that extend angularly, outwardly and then downwardlyfrom the hub 12 toward the forward end of the filter 10. Each arm 18 hasa first arm section 20 which extends angularly outwardly from the hub 12to a shoulder 22, and an outer arm section 24 extends angularly from theshoulder toward the forward end of the filter. The outer arm sections 24are substantially straight lengths with ends which lie on a circle attheir maximum divergence and engage the wall of a vessel at a slightangle (preferably within a range of from ten to forty-five degrees) tocenter the hub 12 within the vessel. For a filter which is to be removedby grasping the hub 12, it is important for the hub to be centered.Normally, there are six wires 18 of equal length extending radiallyoutward from the hub 12 and circumferentially spaced, such as forexample by sixty degrees of arc.

The first filter basket section 14 is the primary filter and can includeup to twelve circumferentially spaced straight wires 26 formingdownwardly extending legs which tilt outwardly of the longitudinal axisof the filter 10 from the hub 12. Six of the wires 26 are shown in FIG.1, and may be of equal length, but normally they are not so that hooks28 at the ends of the wires will fit within a catheter without becominginterconnected. The wires 26 are preferably much longer than the wires18, and have tip sections which are uniquely formed, outwardly orientedhooks 28 which lie on a circle at the maximum divergence of the wires26. There may be from three to twelve of the wires 26 formed with hooks28, although in some instances, the wire arms 18 may include similarlyformed hooks at the free ends thereof. The wires 26, in their expandedconfiguration of FIG. 1, are at a slight angle to the vessel wall,preferably within a range of from ten to forty-five degrees, while thehooks 28 penetrate the vessel wall to anchor the filter againstmovement. The wires 26 are radially offset relative to the wires 18 andmay be positioned halfway between the wires 18 and also may becircumferentially spaced by sixty degrees of arc as shown in FIG. 4.Thus the combined filter basket sections 14 and 16 can provide a wirepositioned at every thirty degrees of arc at the maximum divergence ofthe filter sections. With reference to the direction of blood flow shownby the arrow in FIG. 1, the filter section 14 forms a concave filterbasket opening toward the leading end of the filter 10 while the filtersection 16 forms a concave filter basket opening toward the leading endof the filter 10 downstream of the filter section 14.

The structure of the hooks 28 is important. As in the case of hooksformed on the legs of previously known permanent vena cava filters,these hooks 28 penetrate the vessel wall when the filter 10 is expandedto anchor the filter in place and prevent filter migrationlongitudinally of the vessel in either direction. However, when thesehooks are implanted and subsequently covered by the endothelium layer,they and the filter can be withdrawn without risk of significant injuryor rupture to the vena cava. Minor injury to the vessel wall due to hookwithdrawal such as damage to the endothelial layer or local vena cavawall puncture is acceptable. However, previous filters with rigidanchoring hooks could not be withdrawn without causing unacceptablevessel tearing or local hemorrhage.

With reference to FIGS. 1 and 2, each hook 28 is provided with ajuncture section 30 between the curvature of the hook and the leg 26 (orarm 18) to which the hook is attached. This juncture section isconsiderably reduced in cross section relative to the cross section ofthe leg 26 (or arm 18) and the remainder of the hook. The juncturesection is sized such that it is of sufficient stiffness when the legs26 (or arms 18) are expanded to permit the hook 28 to penetrate the venacava wall. However, when the hook is to be withdrawn from the vesselwall, withdrawal force to which the hook is subjected will cause flexurein the juncture section 30 so that the hook moves toward a positionparallel with the axis of the leg 26 (or arm 18) as shown in brokenlines in FIG. 2. With the hook so straightened, it can be withdrawnwithout tearing the vessel wall leaving only a small puncture.

With reference to FIG. 3, it will be noted that the entire hook 28 canbe formed with a cross section throughout its length which is less thanthat of the leg 26 (or arm 18). This results in straightening of thehook over its entire length in response to a withdrawal force. Thiselasticity in the hook structure prevents the hook from tearing thevessel wall during withdrawal.

As previously indicated, while it is possible that the filter could bemade from ductile metal alloys such as stainless steel, titanium, orelgiloy, it is preferable to make it from nitinol. Nitinol is a lowmodulus material which allows the arms and legs of the device to bedesigned to have low contact forces and pressures while still achievingsufficient anchoring strength to resist migration of the device. Theforce required to cause opening of the hooks 28 can be modulated to thetotal force required to resist filter migration. This is accomplished bychanging the cross sectional area or geometry of the hooks, or bymaterial selection.

In addition to temperature sensitivity, nitinol, when in the temperatureinduced austenitic state, is also subject to stress sensitivity whichcan cause the material to undergo a phase transformation from theaustenitic to the martensitic state while the temperature of thematerial remains above the transition temperature level. By reducing aportion or all of the cross sectional area of the hooks 28 relative tothat of the legs 26 (or arms 18), stress is concentrated in the areas ofreduced cross section when longitudinal force is applied to the hub 12in the direction of the trailing end of the filter to remove the filter,and the hooks become elastic and straighten. Thus the hooks, whetherformed of nitinol, spring metal or plastic, are designed to bend towarda more straight configuration when a specific hook migration force isapplied and spring back to their original shape once the hook migrationforce has been removed. The force or stress which is required to deformthe hook can be correlated to the force applied to each hook of thedevice when it is fully occluded and the blood pressure in the vessel isallowed to reach 50 mmHg. This force is approximately 70 gms on each legof a six leg device for 50 mmHg. pressure differential in a 28 mmvessel. The desired total migration resistance force for the filter isdesirably 420 gms, and more legs 26 with hooks 28 can be added to lowermaximum migration force for each hook. The load on the filter would becorrespondingly smaller in vessels of smaller diameter. The object is tohave the hook perform as an anchoring mechanism at a predeterminedfilter migration resistance force within a range of 10 mmHg up to 120mmHg. Having maintained its geometry at a predetermined filter migrationresistance force within this range, the hook should begin to deform inresponse to a higher force applied in the direction of the filtertrailing end and release at a force substantially less than that whichwould cause damage to the vessel tissue. It is the ability of the hookto straighten somewhat that allows for safe removal of the device fromthe vessel wall.

After the filter 10 has remained in place within a vessel for a periodof time in excess of two weeks, the endothelium layer will grow over thehooks 28. However, since these hooks, when subjected to a withdrawalforce become substantially straight sections of wire oriented at a smallangle to the vessel wall, the filter can be removed leaving only six pinpoint lesions in the surface of the endothelium. To accomplish this, acatheter or similar tubular unit is inserted over the hub 12 and intoengagement with the arms 18. While the hub 12 is held stationary, thecatheter is moved downwardly forcing the arms 18 downwardly, andsubsequently the arms 26 are engaged and forced downwardly therebywithdrawing the hooks 28 from the endothelium layer. Then the hub 12 isdrawn into the catheter to collapse the entire filter 10 within thecatheter. When the filter is formed from shape memory material, coolingfluid can be passed through the catheter to aid in collapsing thefilter.

The primary objective of the hooks 28 is to ensure that the filter doesnot migrate during normal respiratory function or in the event of amassive pulmonary embolism. Normal inferior vena cava (IVC) pressuresare between 2-5 mmHg. An occluded IVC can potentially pressurize to 35mmHg below the occlusion. To ensure filter stability, a 50 mmHg pressuredrop across the filter may therefore be chosen as the design criteriafor the filter migration resistance force for the removable filter 10.When a removal pressure is applied to the filter that is greater than 50mmHg, the hooks 28 will deform and release from the vessel wall. Thepressure required to deform the hooks an be converted to force by thefollowing calculations.

-   -   Since 51.76 mm Hg=1.0 pounds per square inch (psi)    -   50 mm Hg=0.9668 psi    -   For a 28 mm vena cava

$A = {{\frac{\pi}{4}\mspace{11mu}(28)^{2}\mspace{14mu}{mm}^{2}} = {{615.4\mspace{14mu}{mm}^{2}} = {0.9539\mspace{14mu}{inches}^{2}}}}$

-   -   Migration force is calculated by:

$\begin{matrix}\begin{matrix}{P = \frac{F}{A}} & \; & \; & {F = {P \times A}}\end{matrix} \\{{0.9668\mspace{14mu}{psi} \times 0.9539\mspace{14mu}{inches}^{2}} = {{0.9223\mspace{14mu}{pounds}} = {418.7\mspace{14mu}{grams}}}}\end{matrix}$

It is important to recognize that as vena cava diameter increases sodoes the force required to resist 50 mmHg of pressure.

Depending on the number of filter hooks, the strength of each can becalculated. For a device that has six hooks:

$\begin{matrix}{{{Hook}\mspace{14mu}{Strength}} = \frac{{Filter}\mspace{14mu}{Migration}\mspace{14mu}{Resistance}\mspace{14mu}{Force}}{{Number}\mspace{14mu}{of}\mspace{14mu}{Hooks}}} \\{= \frac{418.7}{6}} \\{= {69.7\mspace{14mu}{grams}}}\end{matrix}$

Each hook must be capable of resisting approximately 70 grams of forcefor the filter 10 to resist 50 mmHg pressure gradient in a 28 mm vessel.

To prevent excessive vessel trauma the individual hook needs to berelatively weak. By balancing the number hooks and the individual hookstrength, minimal vessel injury can be achieved while still maintainingthe 50 mmHg pressure gradient criteria, or some other predeterminedpressure gradient criteria within a range of from 10 mmHg to 120 mmHg.

Referring to FIG. 5, the legs 26 may be angled outwardly from a shoulder30 adjacent to but spaced from the outer end of each leg. When the legsare released from compression in a catheter or other tube into a bodyvessel, this bend in each leg insures that the hooks 28 are, in effect,spring loaded in the tube and that they will not cross as they aredeployed from the tube. Since the legs angle outwardly from theshoulders 30, the hooks 28 are rapidly deployed outwardly as theinsertion tube is withdrawn.

The filter delivery unit 32 is adapted to deliver the filter 10 througha catheter or delivery tube 34 to a precise, centered position within abody vessel. The filter delivery unit includes a handle 36 at one end,and an elongate pusher wire 38 extends outwardly from the handle 36. Atthe free end of the pusher wire is an enlarged filter engaging pusherpad 40.

The elongate pusher wire 38 is preferably formed of superelasticmaterial and may be formed of thermally responsive shape memorymaterial, such as nitinol. The pusher wire includes sections 42, 44 and46 which progressively decrease in cross section beginning at the handle36. The temperature transformation level of the pusher wire is such thatwhen the wire is encased in a catheter or delivery tube, it remains in amartensitic state and is therefore somewhat pliable and flexible so thatit can conform to curvatures in a catheter or delivery tube which passesthrough a body vessel. As the delivery tube is withdrawn, bodytemperature causes the exposed portions of the pusher wire to assume themove rigid austenitic state for filter positioning.

A slotted spline 48 is secured to the pusher wire 38 between thesections 44 and 46. The pusher pad is provided with a plurality ofspaced, peripherally arranged, longitudinally extending grooves 50 ofsufficient number to individually receive the legs 26 of a filter 10.The spline is spaced from the pusher pad 40 for a distance less than thelength of the filter legs 26 so that the legs can be received in thegrooves 50 when the pusher pad engages the filter hub 12 as shown inFIG. 8. It will be noted that the pusher wire section 46 is reduced incross section at 52 adjacent to the spline 48.

To load the filter delivery unit 32 to insert a filter 10 into a bodyvessel, the pusher wire section 46 is inserted from the leading end ofthe filter 10 under the arms 18 and legs 26 until the pusher pad 40engages the underside of the hub 12 at the apex of the filter as shownin FIG. 8. Then the legs 26 of the filter, two being shown for purposesof illustration in FIG. 8, are inserted into the grooves 50 in thespline, and the arms 18 are spirally wrapped around the spline. Thepusher wire, with the filter in place, is inserted into a catheter ordelivery tube 34. When the catheter or delivery tube with the filter 10is at a desired location within a body vessel, it is removed from aroundthe delivery unit and filter to expose the filter. First the hub 12 ofthe filter is exposed and then the pusher wire section 46 emerges. Whenthe pusher wire is formed of thermal shape memory material, theemergence of wire section 46 causes this section, with the exception ofthe portion of reduced cross section 52, to transform to the austeniticstate and to become more rigid. As the filter pad 48 emerges, thecentering arms 18 of the filter 10 are exposed and released andtransform to the austenitic state to achieve radial expansion outwardlytoward the vessel wall. If the filter is not centered in the vessel,some of the arms 18 will engage the vessel wall and apply stress to thereduced cross section portion 52 of the pusher wire section 46. Stresscauses this portion 52 to remain in the flexible martensitic state, andthe pusher wire section 46 will pivot at the portion 52 to permit radialmovement of the spline 40 in all directions to aid the arms 18 incentering the filter 10 within the vessel. Thus the portion 52 providesa directional hinge for centering the filter.

With the filter centered, the legs 26 are exposed and expand radially toengage the vessel wall and anchor the filter against migration. Thepusher wire and catheter or delivery tube are now withdrawn from thebody vessel.

When the pusher wire is formed of flexible material which is not athermal, shape memory material, the reduced cross sectional portion 52to the pusher wire section 46 has greater flexibility than the remainderof the pusher wire and thus forms a flexible, directional hinge to aidin centering the filter in the manner previously described.

The invention claimed is:
 1. A method of recovering a blood filterimplanted in a blood vessel, the blood filter having a plurality of legsthat extend from a hub and radially spaced about a longitudinal axis,each of the legs having a hook coupled to a wall of the blood vessel,the hook having a curved profile, the method comprising: deforming thehook of each of the legs from the curved profile towards a generallystraight profile by applying a force greater than 70 grams to each legin a direction along the longitudinal axis towards the hub to separatethe hook of the blood filter from the wall of the blood vessel, the hookhaving an elasticity greater than an elasticity of its correspondingleg; and forcing the plurality of legs to move towards the longitudinalaxis.
 2. The method of claim 1, wherein the applying comprises applyinga force greater than 420 grams to the hub to separate the hooks of theblood filter from the wall of the blood vessel of about 28 millimeters.3. The method of claim 2, wherein the deforming comprises bending eachof the hooks toward a generally straight configuration to release a tipof the hook from the blood vessel wall.